Download Basic Modes of Heat Transfer

Application Note 102
LED-Driver Thermal
Design Considerations
in LED-Based Fixtures Part 2
Basic Modes of Heat
Transfer
Material
Air, atmosphere
(gas)
Aluminum
Copper
Gold
Iron
Silver
Zinc Zn
Plastics,
foamed
Plywood
Polycarbonate
Abstract:
Heat transfer can occur in three different modes:
conduction, convection and radiation. The
purpose of this application note is not to
bombard the reader with a ton of equations.
However, a fixture designer must be well aware
of the controlling parameters in the equations in
each heat transfer mode. Understanding and
utilizing the key parameters in the equations will
determine the success of the thermal design,
hence the performance, cost and reliability of the
fixture.
Thermal Conductivity
Normalize
W/(m K)
d to Al
Normalized
to Air
0.024
205
401
310
80
429
116
8541.7
1.0
0.5
0.7
2.6
0.5
1.8
1.0
8541.7
16708.3
12916.7
3333.3
17875.0
4833.3
0.03
0.13
0.19
6833.3
1576.9
1078.9
1.3
5.4
7.9
Table 1. Thermal Conductivity of Some Common
Materials
Conduction:
Conduction is the most common method of heat
transfer from a heat source, such as an LED and
a driver, to the heatsink in LED fixtures.
Therefore, it is important to go through this
concept and the related equation in more detail.
Thermal conduction happens when two solid
materials that are at two different temperatures
are in direct contact, e.g. an Aluminum heatsink
and a driver. Please see Figure 1 for an example
of a driver making contact with a heatsink. For all
practical purposes, metals are used for
heatsinking purposes since they have very good
thermal conductivity. Please see Table 1 for
thermal conductivity of multiple common
materials. For more details on thermal
conductivity discussion please refer to the
“Practical Thermal Design Considerations”
application note.
Aluminum is the most commonly used metal for
the heatsink and mechanical body or chassis of
an LED based fixture because of its lower cost
compared to other higher heat conductive
materials such as copper and silver. Although
some high thermal conductivity plastic material is
also used for this purpose, metals like aluminum,
and to a much lesser degree copper, are used in
the overwhelming majority of LED lighting
applications. Various grades of aluminum will
have somewhat different thermal conductivity.
Figure 1. Conduction Mode Heat Transfer: Driver
Making Direct Contact with the Heatsink
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January 2017
Application Note 102
Below Equation 1 shows the amount
transferred heat in conduction mode.
of
Qcond = -k A (ΔT / Δx)
Equation 1: Conduction Mode Heat Transfer
where:
Qcond is the amount of heat transferred through
conduction (W)
k is the thermal conductivity of the material (W/m
K)
A is the contact surface area of the material
through which the heat flows (m2)
ΔT is the temperature gradient across the
material (°C)
Δx is the distance the heat must travel (m)
Figure 2. Convection Mode Heat Transfer:
Heatsink Transferring Heat to the Ambient Air
Equation 2 below describes the amount of heat
transferred in convection mode.
Qconv = h A ∆T
Equation 2: Convection Mode Heat Transfer
The parameters of the above equation are very
important for thermal design and must be fully
understood by any LED fixture designer. The
above equation simply states that the higher the
conductivity of the material, the larger the
contact area, and the thinner the material is, the
more heat can be transferred in a given design.
Out of all of the parameters in Equation 1, the
easiest to control is the A. Therefore, it is very
important to increase the contact surface as
much as possible, and take advantage of it by
making a good thermal contact with the heatsink
to transfer the most amount of heat. The second
parameter that one can easily control is Δx. The
less distance the heat needs to travel the better
the thermal conduction. Thermal conductivity, K,
is going to be fixed by the type and grade of the
material used. Most often using basic heatsink
grade aluminum yields the most optimum system
solution since exotic materials and specialized
grades tend to increase the system cost
dramatically.
where:
Qconv is the amount of heat transferred through
convection (W)
h is the heat transfer coefficient (W/m2K)
A is the surface area (m2)
ΔT is the temperature gradient across the
material (°C), difference between the surface
temperature and ambient air temperature
Based on Equation 2, the higher the heat transfer
coefficient, and the larger the surface area
(typically heatsink fin surface) the higher the
amount of heat transferred in convection mode
for a given design. Needless to say, the hotter the
heatsink the more heat will be transferred to the
ambient air. This equation is a bit tricky because
h, heat transfer coefficient, encompasses several
parameters, namely airflow. It is extremely
important to understand air flow and making
proper design considerations for airflow since it
plays a major role in convection. To give the
reader an idea for natural convection, h is roughly
between 5 to 20 W/m2K. If there is a fan in the
system, depending on the speed, h could be as
high as 120 W/m2K. Based on the above
examples, a thermal design can be over 10 times
more efficient if there is good airflow around the
heatsink fins. This is not to suggest or promote
including a fan in every design. On the contrary,
fan usage should be limited to only a very small
percentage of designs where a passive heatsink
design won’t work under certain size or cost
Convection:
Convection is the heat transfer mode from a solid
material to fluid (gas or liquid). Understanding
convection mode is also very important since
most heatsinks and fins operate in this mode,
basically transferring the heat from the heatsink
to ambient air.
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Application Note 102
constraints. This topic will be covered in the “Fin
Design” application note in great detail. In
passive heatsink designs, a great deal of
attention must be paid to increasing air flow as
much as possible. There is a diminishing point
where adding extra fin surface area doesn’t help
the thermal performance because the air flow is
restricted. Most often having good airflow yields
better results than increasing the size and cost of
the heatsink. Promoting good airflow in the
heatsink and fixture chassis design is a critical
factor for good fixture thermal performance. A
good airflow design in a passive heatsink system
will often reduce the cost and size of the system.
temperature in Kelvin degrees on the blackbody
locus defines the CCT (Correlated Color
Temperature) of LEDs.
As shown in Equation 3, emissivity of the material
and the surface area have a direct relationship
with the radiated heat transfer. Please note that
the Boltzmann constant σ (5.67 x 10-8 W/m2K4) is
an extremely small number, therefore when
multiplied by other parameters in Equation 3, it
results in a minuscule figure, hence the reason
for radiation heat transfer mode being ignored for
most lighting fixture thermal calculations.
Qrad = ε σ A (Ts4- Tf4)
Equation 3: Radiation Mode Heat Transfer
Radiation:
Radiation is the third and least important mode of
heat transfer in LED lighting fixtures. Radiation
heat transfer is done through electromagnetic
radiation which is generated by the thermal
motion of particles inside the materials. For all
practical purposes, radiation may be ignored
since it is a very small percentage of the heat
transfer for LED based lighting fixture designs. As
a rule of thumb, radiation becomes more
important only for very high power concentrated
designs where the temperature on the heatsink is
typically above100°C.
To visualize the concept of radiation, the sun is
the simplest example, as shown in Figure 3.
Energy from the surface of the sun is radiated
from an extremely long distance to earth without
the need for air to transfer the heat.
where:
Qrad is the amount of heat transferred through
radiation (W)
ε is the emissivity of the surface (dimensionless)
σ is the Stefan-Boltzmann constant (5.67 x 10-8
W/m2K4)
A is the surface area (m2)
Ts is the surface temperature of the material (°C)
Tf is the fluid (e.g. ambient air) temperature of the
medium (°C)
Based on the heat transfer modes described
above, for an optimum thermal design the
following general guidines should be seriously
considered.
- The contact surface between the driver
and the heat sink is as large as possible.
- Make good thermal contact between the
driver and the heat sink.
- The heat sink bulk material is as thin as
possible.
- Increase heat sink fin area while making
sure the fin density doesn’t restrict air
flow.
- Allow as much air flow as possible.
In conclusion, a fixture designer is advised to
fully understand the three modes of heat transfer.
One may not be very concerned about the
equations and detailed thermal calculations,
however it is imperative to understand, fully
consider and utilize the critical parameters of the
above heat transfer mode equations, like surface
area, thermal conductivity and airflow in order to
come up with a good thermal design. Conduction
and convection heat transfer modes are well
utilized in a lighting fixture, however for all
practical purposes the radiation heat transfer
Figure 3. Example of Radiation Mode Heat
Transfer
Another good example of radiated heat is an iron
rod heated over 1000°C. The radiated color of
the rod has a direct relationship to the
temperature that it is heated up to. In fact, the
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Application Note 102
mode may be ignored since it is a very tiny
percentage of the total heat transferred.
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product specifications, please see the data sheets available at www.erp-power.com. For warranty information, please contact ERP Sales at [email protected].
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